[Glitch] Add media description as title to links of unknown media attachments

Port front-end changes from 49ebda4d49 to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/styles/components/composer.scss

@@ -370,6 +370,7 @@
     border-radius: 4px;
     height: 140px;
     width: 100%;
+    background-color: $base-shadow-color;
     background-position: center;
     background-size: cover;
     background-repeat: no-repeat;